Method for the purification of trichlorosilane



United States Patent 3,516,803 METHOD FOR THE PURIFICATKON OFTRICHLOROSILANE Herbert J. Moltzan and De Winn Fytfe, Dallas, Tex., as-

signors to Texas Instruments Incorporated, Dallas, Tex., a corporationof Delaware No Drawing. Filed Oct. 6, 1966, Ser. No. 584,646 Int. Cl.C01b 33/08 US. Cl. 23-366 3 Claims ABSTRACT OF THE DISCLOSURE A methodof purifying trichlorosilane by heating the impure trichlorosilane to atemperature just below its boiling point, maintaining this temperaturefor a sufiicient period of time to allow evaporation of trichlorosilanevapors, and subsequently condensing the purified vapors and collectingthe condensate. This process is an improvement over conventionaldistillation techniques, since the latter cause bubbling of thetrichlorosilane and corresponding entrainment of impurity-containingliquid droplets in the trichlorosilane distillate.

This invention relates to the purifiication of trichlorosilane, and moreparticularly to the purification of trichlorosilane containing volatilecompounds of elements, such as boron and phosphorus, that formelectrically active carriers in silicon.

Trichlorosilane (SiHC1 is a silicon containing material which can bereadily reduced for forming silicon bodies used in the manufacture ofelectronic elements, such as transistors and the like. For such uses, itis of considerable advantage to have a relatively pure trichlorosilane.The contaminants or impurities of principal concern are those impuritieswhich greatly reduce the resistivity of the silicon formed by reductionof the trichlorosilane. For example, one part per billion, atomic ratio,of a P-type impurity, i.e., a group III metal such as boron, is believedto reduce the resistivity of pure silicon from about 230,000ohm-centimeters (theoretical) to 285 ohmcentimeters. N-type impurities(group V and VI metals) also affect the electrical properties. Forexample, one part per billion, atomic ratio, of an N-type impurity suchas phosphorus is believed to reduce the resistivity of pure silicon fromabout 230,000 ohm-centimeters (theoretical) to 85 ohm-centimers, all ofwhich is known to those skilled in the art.

The principal impurities affecting electrical properties of silicon areboron (P-type) and phosphorus (N-type) and eiforts have been made toremove these impurities by distillation. While the exact nature of theseimpurities as they appear in commercial trichlorosilane is unknown,several assumptions can be made. Boron is probably present as borontrichloride (BCl This material boils at 12.5 C. and would be expected tocome over during the early part of a distillation since the boilingpoint of trichlorosilane is 31.8 C. Phosphorus, however, can be presentas phosphorus trichloride (PCl phosphine (PH diphosphine (P H orpossibly as an intermediate phosphorushydrogen-halide compound. Theintermediates are not characterized in the literatures but have beenpostulated. Phosphorus trichloride boils at 75.5 C. and would beexpected to remain in the bottoms during distillation. Phosphine, on theother hand, is a colorless gas having a boiling point of 85 C. and wouldvery readily distill at normal trichlorosilane boiling temperature. Thepostulated intermediate phosphorus-hydrogen-halide compounds would haveproperties between phosphorus trichloride and phosphine. Diphosphine hasbeen reported to boil between 51 C. and 56 C. It does 3,516,803 PatentedJune 23, 1970 decompose, however, to phosphine and an intermediate (PH)which eventually reduces further to phosphine and phosphorus. Thephosphine (PH would, as mentioned, because of its low boiling point goover with the distillate. Diphosphine decomposes to form phosphine (PHat a rate that increases with increasing temperatures and the phosphineso formed will be found in the distillate.

Distillation techniques have also been applied for the removal fromtrichlorosilane of volatile compounds of elements that form electricallyactive carriers in silicon. These techniques are only partiallysuccessful, however, in that small amounts of impurities remain in thetrichlorosilane, and these small amounts are suflicient to affect theelectrical characteristics of the silicon metal that is produced.Distillation does not completely remove these impurities because of theentrainment of very minute amounts of liquid in each gas bubble or inthe gas phase formed during the distillation. These minute amounts ofliquid present in the gas or vapor phase originate from the crudetrichlorosilane being distilled and are consequently rich in impuritycontent. Therefore, these contaminants being entrained, are notseparated by the distillation and thereby result in a distilledtrichlorosilane still containing small but significant amounts ofimpurities that will affect the electrical characteristics of silicon.

Efiorts have also been made at removing the impurities by chemicaltreatment of the trichlorosilane, but such efforts are expensive andgenerally unsatisfactory due to the possibility of contamination fromthe treating agent.

The present invention is an improved method for purifyingtrichlorosilane and may generally be described as including the steps ofheating the impure trichlorosilane to a temperature just below itsboiling point, condensing the vapors produced and collecting thecondensate.

The method of the present invention lessens or eliminates thepossibility of diphosphine (P H decomposition to phosphine (PH becauseof close temperature control and the elimination of hot spots which arepresent in any distillation. It also eliminates the possibility ofentrainment of impurities in the vapors which emanate from adistillation column since there is no boiling of the trichlorosilane.

The method of the present invention may be practiced by use of varioustype equipment, but because of the nature of trichlorosilane, theequipment is preferably constructed of quartz, Teflon or a mild steel,though other materials are suitable. To illustrate the'etfectiveness ofthe method of the invention and its adaptability to various typesystems, the following examples are given.

CLOSED QUARTZ SYSTEM Example 1 A quartz flask was connected at the topby a piece of quartz tubing to a Teflon bottle immersed in Dry Ice andwater. All equipment which was to contact the trichlorosilane was etchedwith hydrofluoric acid and washed with deionized water. The flask wascharged with 2150 milliliters of an impure mixture of percent by volumetrichlorosilane and 40 percent by volume silicon tetrachloride (SiClwhich was heated to and maintained at 31 C. in a water bath. A quantityof 450 milliliters of condensate was collected and the analysis of thefeed, condensate, and residue is tabulated below:

Example 2 P-type N-type impurity impurity (parts per (parts per billion)billion) Feed 0. 11 1. 93 Condensate- 0. 03 0. 13 Residue O. 20 0. 83

VENTED QUARTZ SYSTEM Example 3 The vented quartz system utilized aquartz flask which is connected to a quartz condenser having a jacketthrough which flowed water maintained between 8 C. and 10 C. Thecondenser discharged into a Teflon bottle cooled with Dry Ice and waterwhich in turn was connected at the top by Teflon tubing to a secondTeflon bottle cooled by Dry Ice and acetone. The quartz flask wascharged with 3304 grams of impure trichlorosilane which was maintainedbetween 31 C. and 31.7 C. until 3075 grams of condensate had beencollected with the analysis of the feed, condensate and residuetabulated below:

P-type N-type impurity impurity (parts per (parts per billion) billion)Feed 0. 39 O. 72 Condensate 0. 16 0. 20 Residue 0. 10 5. 95

Example 4 P-type N-type impurity impurity (parts per (parts per billion)billion) Feed 0.25 O. 30 Condensate 0. 10 0. l8 Residue. 1. 46 2. 87

STEEL TANK SYSTEM A horizontal steel tank provided with a weir thatdivided the tank into two chambers was employed in Examples 57.Communicating with the first chamber was a trichlorosilane charge lineand positioned within the chamber below the top of the weir was a coilof tubing for heating the charged material. A drain line was providedfor emptying the first chamber of residue. Within the second chamber,which communicated with the first over the top of the weir was a coolingcoil for condensing vapor generated in the first chamber. The secondchamber through suitable tubing emptied condensate and any uncondensedvapor into a first Teflon bottle situated in a Dry Ice and water bath.The first Teflon bottle contained a vent line to a second Teflon bottlesituated in a Dry Ice and acetone bath. The second Teflon bottle wasvented to the atmosphere.

Example 5 The first chamber of the tank was charged with approximatelythree liters of impure trichlorosilane. Water at approximately 36 C. wascirculated through the coil in the first chamber in suflicient quantityto maintain the charged trichlorosilane at 29 C. Hydrogen at the rate of1500 milliliters per minute was swept through the first 4 chamber intothe second chamber and from the second chamber through the first andsecond bottles to expedite the evaporation of trichlorosilane, which waspercent (by weight) vaporized and the impurity analysis is tabulatedbelow:

The same equipment, conditions and charge as described in Example 5 wereused to get approximately 90 percent (by weight) condensate, but thehydrogen was swept through the system at one liter per minute andpressure in the tank was maintained between 5 and 6 p.s.i.g. and thetemperature raised to 37 C. An analysis of the feed, condensate andresidue is tabulated below:

Each of the above analyses was performed by zone refining a silicon rodformed from the feed, condensate or residue, as the case may be,followed by a resistivity determination from which the concentration ofimpurities may be calculated, which technique is known to those skilledin the art.

Since it is believed that the initial condensate formed is higher inimpuritie due to the presence of phosphine, a purer product may beobtained by discarding the first 5-25 percent by volume or weight of thecondensate, or if desired the trichlorosilane may be initially passedthrough a distillation column to remove 5-25 percent of the feed fromthe top of the column before feeding the balance of the feed toequipment such as described above.

As will be obvious to those skilled in the art after a reading of theproceeding material, the temperature of evaporation may be varieddepending upon the pressure of the system, i.e., the higher thepressure, the higher the temperature at which the impure trichlorosilanemay be maintained without boiling. For example, in some installations itmay be desirable to maintain the pressure of the system at an absolutepressure of between 1 and 4 atmospheres. At such pressure, the boilingpoint of the trichlorosilane will be between about 31.8 C. and 70 C.,and the temperature of the heating coils must be accordingly raised. Ofcourse, higher pressures and temperatures may be employed.

It is also preferred that the difl'erential temperature between theheating means employed and the trichlorosilane be maintained at lessthan 10 C. and preferably between /z C. and 5 C. to avoid the formationof hot spots in the liquid which promote the decomposition ofdiphosphine.

In many applications, it will not be necessary to condense thetrichlorosilane vapor. For example, the vapor may be directed from theevaporating chamber into a high temperature furnace where it is reducedby a hydrogen stream to form virtually pure silicon.

Also nitrogen, argon or other inert gas, as well as hydrogen, may beused to expedite the evaporation rate.

While rather specific term have been used in describing various examplesof the invention, they are not intended, nor should they be construed,as limitations on the invention as defined by the claims.

What is claimed is:

1. A method of purifying liquid phase trichlorosilane containing notmore than 96 parts per billion N-type impurity, including diphosphine,which comprise the steps:

placing said liquid phase trichlorosilane containing diphosphine in afirst additive-free chamber containing a heating means;

heating said trichlorosilane containing diphosphine to a temperaturejust below its boiling point, while at all times maintaining thetemperature difference between said heating means and thetrichlorosilane at less than 10 C.;

removing purified vapors of trichlorosilane from the liquid phase;

sweeping said vapors from said first chamber into a second chamber bymeans of an inert gas having a boiling point lower than the boilingpoint of said trichlorosilane, said second chamber containing condensingmeans;

condensing said 'vapor within said second chamber to produce a purifiedliquid trichlorosilane; and

venting said inert gas from the condensate formed in said secondchamber.

2. The method defined by claim .1 wherein the pressure maintained onsaid impure trichlorosilane is atmospheric and the temperature ismaintained between 29 C. and 31.8 C.

3. The method defined by claim 1 wherein the pressure on said impuretrichlorosilane is maintained between about 1 and 4 atmospheres and thetemperature is maintained between /2 C. and 10 C. less than the boilingpoint of trichlorosilane at said pressure.

References Cited UNITED STATES PATENTS 3,020,128 2/1962 Adcock et al.23223.5 3,188,168 6/1965 Bradley 23-223.5 X

FOREIGN PATENTS 906,617 9/ 1962 Great Britain. 929,696 6/ 1963 GreatBritain. 1,072,227 12/ 1959 Germany.

20 OSCAR R. VERTIZ, Primary Examiner G. O. PETERS, Assistant ExaminerUS. Cl. X.R. 23205

